In silver halide photography one or more silver halide emulsion layers are usually coated on a single side of a support. An important exception is in medical radiography. To minimize X-ray dosage received by a patient silver halide emulsion layers are commonly coated on both sides of the support. Since silver halide emulsions are relatively inefficient absorbers of X-radiation, the radiographic element is positioned between intensifying screens that absorb X-radiation and emit light. Crossover exposure, which results in a reduction in image sharpness, occurs when light emitted by one screen passes through the adjacent emulsion layer and the support to imagewise expose the emulsion layer on the opposite side of the support. Loss of image sharpness is a result of light spreading in passing through the support. In radiographic applications in which the level of X-ray exposure can be increased wihout injury to the subject, as in nondestructive testing of materials, crossover can be avoided by coating on a single side of a support.
A great variety of regular and irregular grain shapes have been observes in silver halide photographic emulsions intended for black-and-white imaging applications generally and radiographic imaging applications specifically. Regular grains are often cubic or octahedral. Grain edges can exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical or near spherical thick platelets, as described, for example by Land U.S. Pat. No. 3,894,871 and Zelikman and Levi Making and Coating Photographic Emulsions, Focal Press, 1964, page 223. Rods and tabular grains in varied portions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has varied during precipitation, as occurs, for example in single-jet precipitations.
Tabular silver bromide grains have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The aspect ratio--that is, the ratio of diameter to thickness--of tabular grains is substantially greater than 1:1. High aspect ratio tabular grain silver bromide emulsions were reported by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125.
From 1937 until the 1950's the Eastman Kodak Company sold a Duplitized.RTM. radiographic film product under the name No-Screen X-Ray Code 5133. The product contained as coatings on opposite major faces of a film support sulfur sensitized silver bromide emulsions. Since the emulsions were intended to be exposed by X-radiation, they were not spectrally sensitized. The tabular grains had an average aspect ratio in the intermediate range of from about 5 to 7:1 and were relatively thick. The tabular grains accounted for greater than 50% of the projected area while nontabular grains accounted for greater than 25% of the projected area. The emulsion having the highest average aspect ratio, chosen from several remakes, identified below as Control 1, had an average tabular grain diameter of 2.5 microns, an average tabular grain thickness of 0.36 micron, and an average aspect ratio of 7:1. In other remakes the emulsions contained thicker, smaller diameter tabular grains which were of lower average aspect ratio.
Although tabular grain silver bromoiodide emulsions are known in the art, none exhibit a high average aspect ratio. A discussion of tabular silver bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographic Emulsions", Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
Bogg, Lewis, and Maternaghan have recently published procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounded by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of {100} crystal faces. In the example reported the average edge length of the grains was 0.93 micron and the average aspect ratio 2:1. Thus the average grain thickness was 0.46 micron, indicating thick tabular grains were produced. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, causing the seed crystals to increase in size by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwalt ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994, 4,184,877, and 4,184,878, U.K. Pat. No. 1,570,581, and German OLS publication Nos. 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed.) Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. It appears from repeating examples and viewing the photomicrographics published that average tabular grain thicknesses were greater than 0.40 micron. Japanese patent Kokai No. 142,329, published Nov. 6, 1980, appears to be essentially cumulative with Maternaghan, but is not restricted to the use of silver iodide seed grains. Thus, the patents discussed above in this paragraph are viewed as teaching the preparation of silver halide emulsions containing relatively thick tabular grains of intermediate average aspect ratios.
Wilgus and Haefner U.S. Ser. No. 429,420, filed concurrently herewith and commonly assigned, titled HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses high aspect ratio silver bromoiodide emulsions and a process for their preparation.
Kofron et al U.S. Ser. No. 429,407, filed concurrently herewith and commonly assigned, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE EMULSIONS AND PHOTOGRAPHIC ELEMENTS, which is a continuation-in-part of U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions.
Daubendiek and Strong U.S. Ser. No. 429,587, filed concurrently herewith and commonly assigned, titled AN IMPROVED PROCESS FOR THE PREPARATION OF HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS, which is a continuation-in-part of U.S. Ser. No. 320,906, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses an improvement on the processes of Maternaghan whereby relatively thin high aspect ratio tabular grain silver bromoiodide emulsions can be prepared.
Wey U.S. Ser. No. 429,403, filed concurrently herewith and commonly assigned, titled IMPROVED DOUBLE-JET PRECIPITATION PROCESS AND PRODUCTS THEREOF, which is a continuation-in-part of U.S. Ser. No. 320,908, filed Nov. 12, 1981, now abandoned, discloses a process of preparing tabular silver chloride grains which are substantially internally free of both silver bromide and silver iodide. The emulsions have an average aspect ratio of greater than 8:1.
Solberg, Piggin, and Wilgus U.S. Ser. No. 431,913, filed concurrently herewith and commonly assigned, titled RADIATION-SENSITIVE SILVER BROMOIODIDE EMULSIONS, PHOTOGRAPHIC ELEMENTS, AND PROCESSES FOR THEIR USE, which is a continuation-in-part of U.S. Ser. No. 320,909, filed Nov. 12, 1980, now abandoned discloses a high aspect ratio tabular grain silver bromoiodide emulsions wherein a higher concentration of iodide is present in an annular region than in a central region of the tabular grains.
Dickerson U.S. Ser. No. 430,574, filed concurrently herewith and commonly assigned, titled FOREHARDENED PHOTOGRAPHIC ELEMENTS AND PROCESSES FOR THEIR USE, which is a continuation-in-part of U.S. Ser. No. 320,910, filed Nov. 12, 1981, now abandoned discloses producing silver images of high covering power by employing photographic elements containing forehardened thin tabular grain silver halide emulsions.
Mignot U.S. Ser. No. 320,912, filed concurrently herewith and commonly assigned, titled SILVER BROMIDE EMULSIONS OF NARROW GRAIN SIZE DISTRIBUTION AND PROCESSES FOR THEIR PREPARATION discloses high aspect ratio tabular grain silver bromide emulsions wherein the tabular grains are square or rectangular in projected area.
Jones and Hill U.S. Ser. No. 430,092, filed concurrently herewith and commonly assigned, titled PHOTOGRAPHIC IMAGE TRANSFER FILM UNIT, which is a continuation-in-part of U.S. Ser. No. 320,911, filed Nov. 12, 1981, now abandoned, discloses image transfer film units containing tabular grain silver halide emulsions.
Evans et al U.S. Ser. No. 431,912, filed concurrently herewith and commonly assigned, titled DIRECT REVERSAL EMULSIONS AND PHOTOGRAPHIC ELEMENTS USEFUL IN IMAGE TRANSFER FILM UNITS which is a continuation-in-part of U.S. Ser. No. 320,891, filed Nov. 12, 1981, now abandoned, discloses image transfer film units containing tabular grain core-shell silver halide emulsions.
Maskasky U.S. Ser. No. 431,455, filed concurrently herewith and commonly assigned, titled SILVER CHLORIDE EMULSIONS OF MODIFIED CRYSTAL HABIT AND PROCESSES FOR THEIR PREPARATION, which is a continuation-in-part of U.S. Ser. No. 320,898, filed Nov. 12, 1981, now abandoned, discloses a process of preparing tabular grains having opposed major crystal faces lying in {111} crystal planes and, in one preferred form, at least one peripheral edge lying perpendicular to a &lt;211&gt; crystallographic vector in the plane of one of the major surfaces. Thus, the crystal edges obtained are crystallographically offset 30.degree. C. as compared to those of Wey. Maskasky requires that the novel tabular grains be predominantly (that is, at least 50 mole percent) chloride.
Wey and Wilgus U.S. Ser. No. 431,854, filed concurrently herewith and commonly assigned, titled NOVEL SILVER CHLOROBROMIDE EMULSIONS AND PROCESSES FOR THEIR PREPARATION which is a continuation-in-part of U.S. Ser. No. 320,899, filed Nov. 12, 1981, now abandoned, discloses tabular grain silver chlorobromide emulsions in which the molar ratio of chloride to bromide ranges up to 2:3.
Maskasky U.S. Ser. No. 431,855, filed concurrently herewith and commonly assigned, titled CONTROLLED SITE EPITAXIAL SENSITIZATION, which is a continuation-in-part of U.S. Ser. No. 320,920, filed Nov. 12, 1981, now abandoned discloses high aspect ratio tabular grain emulsions wherein silver salt is epitaxially located on and substantially confined to selected surface sites of the tabular silver halide grains.
Abbott and Jones U.S. Ser. No. 430,222, filed concurrently herewith and commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED CROSSOVER, which is a continuation-in-part of U.S. Ser. No. 320,907, filed Nov. 12, 1981, now abandoned, discloses radiographic elements comprised of first and second imging portions separated by an interposed support capable of transmitting radiation to which the second imaging portion is responsive. At least the first imaging portion includes a silver halide emulsion in which high aspect ratio tabular silver halide grains are present. Spectral sensitizing dye is adsorbed to the surface of the tabular grains. Crossover is improved in relation to the imaging characteristics of the radiographic element.